On the PHP-GTK Community Site today, there's a quick new tutorial posted on how to use the Zend Framework (the Zend_XmlRpc_Client component specifically) to work with the geo-nick web service.

Closing the tour of XML-RPC implementations usable with PHP-GTK, the shortest code to access our geo_nick service makes use a higher-level XML-RPC library: the Zend_XmlRpc_Client from the Zend Framework.

They include the code you'll need to get started, including the creation of the client and making the request to the remote service.